AIR BAG STARTING CIRCUIT

PROBLEM TO BE SOLVED: To supply an ignition current continuously only for a necessary time, and surely expand an air bag by providing a coil for laying a mechanical switching element into the closed state by current-carrying in a position magnetically corresponding to the contact of this element, and carrying the current to the coil for a prescribed time when the mechanical switching element is closed. SOLUTION: In a collision, a saving sensor SS1 detects collision in a low acceleration and closes its contact 3a. A microcomputer 12 then transmits a current-carrying signal to a coil current-carrying circuit 11, and the current- carrying circuit 11 also supplies a current to a coil 1 to excite it. Even when an acceleration is added to the saving sensor SS1 to separate a magnet 4 from the contact 3a, the contact 3a of a lead switch 3 is held in the closed state by the magnetic field formed by the coil 1. When a G sensor 91 thereafter detects the collision in a high acceleration, and the microcomputer 12 finally judges the collision, an ignition current controlling transistor Tr is ON, and an ignition element SC is ignited to expand an air bag.